#include <stdio.h>

int main(){

  int i,n,p[5000],count=0;

  scanf("%d",&n);

  for(i=0;i<n;i++)scanf("%d",&p[i]);


  if(p[0] < p[n-1]){

    if(p[n-1]>p[n-2] && p[n-1]!=0){
      count+=(p[n-1]-p[n-2]);
      p[n-1]=p[n-2];
    }
    for(i=n-1;i>=0;i--){
      if(p[i]>0 && i!=0){
	count+=p[i]*2;
	p[i-1]=(p[i-1]-p[i]);
	p[i]=0;
      }else if(p[i]>0){
	count+=p[i];
	p[i]=0;
      }
    }


  }else{
    if(p[0]>p[1] && p[0] !=0){
      count += p[0]-p[1];
      p[0]=p[1];
    }

    for(i=0;i<n;i++){

      if(p[i]>0 && i!=n-1){
	count+=p[i]*2;
	p[i+1]-=p[i];
	p[i]=0;
      }else if(p[i]>0){
	count+=p[i];
	p[i]=0;
      }
    }
  }
  printf("%d\n",count);

  return 0;

}